This invention relates generally to the generation and extraction of pulsed ion beams and more particularly to a low-inductance, inverse reflex tetrode (IRT) which produces an intense, unidirectional beam of ions having a low transverse temperature, operates with an essentially constant impedance, is highly efficient and independent from the value of an applied external magnetic field.
Existing sources of intense ion beams may be categorized as follows: (i) magnetically insulated diodes; (ii) pinched beam diodes; (iii) reflex triodes; and (iv) reflex tetrodes. In magnetically insulated diodes the efficiency is high but the ions must be accelerated perpendicular to an external magnetic field which exceeds the value required to suppress the electron flow. This strong magnetic field influences the orbits of ions and their propagation properties. Pinched beam diodes are characterized by a relatively high efficiency but fail to operate in the presence of an external magnetic field. Reflex triodes require an external magnetic field and their efficiency is relatively low. The efficiency of reflex tetrodes is high but only if immersed in an external magnetic field. In addition, at high power levels, the impedance of both triodes and tetrodes drops almost monotonically during their operation and thus they cannot be coupled to low impedance generators and operate with high efficiency. Finally, in both these two devices the ion beam is extracted through a virtual cathode, which, in general, is neither stationary nor parallel to an anode because the shape and position of a virtual cathode vary with time. As a result of the latter, the ions experience a radial electric field in the accelerating gap (between the anode and virtual cathode) and thus acquire a velocity transverse to the applied magnetic field (beam heating). In some instances an ion beam which is extracted through a virtual cathode is not current-neutralized. Extraction of an ion beam with small divergence and energy spread through a virtual cathode is difficult.
Stephanakis et. al. (Phys. Rev. Letter 37, 1543, 1976) have observed enhanced ion yields in a pinched-beam-diode ion source when an opaque carbon anode is replaced with a semitransparent foil. The slight yield enchancement has been attributed to the reflexing of electrons through the thin anode foil. The predominant mechanism responsible for the ion generation is the pinching of the electron beam. Since the pinching is suppressed by an external magnetic field, the pinched-beam-diode ion source is useful only in those applications that do not require an applied magnetic field.
Similarly, Creedon et. al. (U.S. Pat. No. 4,080,549, Mar. 21, 1978) have proposed a device for relexing electrons and producing a flow of ions which is internal to the device. This device has a very large inductance and thus cannot operate efficiently with low-impedance generators. Also, the device has no means for limiting electron losses within the structure of the device, nor for preventing electron pinched flow, nor any means for extracting an ion beam, particularly a low-transverse-temperature ion beam.